With automatic transmissions known from prior practice, even after an electric transmission control system has been switched off or after a failure of the electric transmission control system it is still possible to continue operating a vehicle, to a limited extent, during a mechanical emergency operation mode of the automatic transmission. During such emergency operation at least one of the respective gear ratios available in the automatic transmission can be engaged as an emergency gear ratio. By virtue of the design of the planetary automatic transmission with power-shiftable friction shifting elements that are preferably acted upon by hydraulic pressure for torque transmission, the emergency gear ratio can be directly engaged even after initiating of the mechanical emergency operating mode. The emergency gear ratio is usually chosen depending on the current speed of the vehicle at the time. The transmission emergency concept, on which emergency operation is based, then depends on the transmission design concerned, and in various transmission systems no satisfactory emergency-running concept can be implemented.
With planetary automatic transmissions that are made with a hydraulic torque converter as the starting element, in the emergency-running mode when the emergency gear is engaged the vehicle can be set in motion again from rest by virtue of the torque converter. After re-starting a drive machine of a vehicle drive-train, as is known it is also provided that in planetary automatic transmissions, a higher emergency gear ratio compared with a previously engaged gear step can be engaged.
In contrast, in vehicles made with a dual clutch transmission with a wet starting clutch, starting processes during the mechanical emergency-running operation of the dual clutch transmission cannot be carried out, since the gear currently engaged in the dual clutch transmission cannot be changed on entry into the mechanical emergency operating mode. For that reason, if the speed of a motor falls below a lower rotational speed threshold or the vehicle speed falls below a certain value, the force flow in the area of the dual clutch transmission is interrupted.
With automated change-speed transmissions having a dry clutch system as well, re-starting of a vehicle from rest after the transition to emergency operation is not possible. This results from the fact that with such transmission systems, on transition to mechanical emergency-running operation the particular gear ratio step engaged in the automated transmission before the beginning of emergency operation cannot be disengaged and it is not possible to engage a desired emergency gear ratio. The motor of the vehicle built with an automated change-speed transmission will stall at the latest when the vehicle comes to rest in the event that the electric transmission control system fails.
Furthermore the possible design of the mechanical emergency-running operation of a transmission device also depends on an operating concept provided in the passenger compartment, by means of which a driver can, in each case, transmit to the transmission device corresponding indications regarding the desired operating status of the transmission device.
With so-termed mechanical operating concepts, the driver is usually provided with hydraulic slides in the transmission controls, with which the driver is able to convey his drive wishes at the time to the transmission. The driver's wish, indicated each time by the driver, regarding an operating condition to be obtained in the transmission device, such as the engagement of a gear for driving forward or in reverse, a parking operating condition in which a drive output is usually blocked mechanically and a force flow in the area of the transmission device is interrupted, or a neutral operating condition in which the drive output can rotate freely and the force flow in the area of the transmission device is interrupted as in the case of the parking operating condition, is determined in part electrically as a function of the position of a selector slide that can be actuated by the driver.
Electrical operating concepts that can be driver-actuated communicate driver's wish specifications to the transmission by means of purely electric signals. By corresponding electro-hydraulic actuators of the transmission device these are then appropriately implemented in accordance with the driver's wish, and the actuation of a parking lock also takes place electro-hydraulically.
Again, in the case of driver-actuated mixed concepts, the driver's wish is communicated to the transmission electrically, whereas the parking lock can be actuated by the driver mechanically.
Furthermore, so-termed electric motor operating concepts are also known, which correspond essentially to mechanical operating concepts and are additionally made with external actors by means of which the driver's wish, communicated in each case by mechanical means in mechanical operating concepts, is translated and implemented by electric motors into an actuation position equivalent thereto.
In general, to make automatic transmissions and particularly also planetary automatic transmissions more economically, shifting elements in the form of power-shiftable frictional shifting elements of the automatic transmissions are to an increasing extent being replaced by non power-shiftable shifting elements such as claw clutches, whereas it is necessary to carry out gear changes in such automatic transmissions without interruption of traction force. This can be done in a simple manner if the non power-shiftable or interlocking shifting element is in each case disengaged for gear changes in the direction of higher gears. In the case of downshifts during which the non power-shiftable shifting elements have to be engaged, the engagement process of a non power-shiftable shifting element is assisted by appropriate action of the motor or with the aid of further, frictional shifting elements not involved in the shifting sequence. In this way it is possible to circumvent the disadvantages of non power-shiftable or interlocking shifting elements which, in contrast to power-shiftable shifting elements, cannot themselves produce any synchronous condition and which, when subjected to torque, sometimes cannot be changed to an open operating condition.
The emergency-running concepts presently known do not enable transmission devices built with both frictional and interlocking shifting elements to be operated to the extent desired for making it possible to continue driving a vehicle after failure of an electric transmission control system, especially starting from when the vehicle is at rest, at least under limited power in order, for example, to make its way to a repair workshop without having to tow the vehicle.